1. Field of the Invention
The present invention relates to an optical modulator used for an optical communication, and in particular, to a Mach-Zehnder optical modulator.
2. Description of the Related Art
For example, an optical waveguide device using an electro-optic crystal such as lithium niobate (LiNbO3), lithium tantalate (LiTaO2) or the like, is formed such that a metallic film is formed on a part of a crystal substrate, to be thermally diffused or to be proton exchanged in benzoic acid after patterning, so that an optical waveguide is formed, and thereafter, an electrode is disposed in the vicinity of the optical waveguide. As one of optical waveguide devices using the electro-optical crystals, there has been known a Mach-Zehnder optical modulator having an optical waveguide structure of branching interference type.
FIG. 7 is a top view showing a configuration example of a conventional Mach-Zehnder optical modulator. In this Mach-Zehnder optical modulator, an optical waveguide 110 is formed such that a titanium (Ti) film is formed on a substrate 101 to be patterned into a Mach-Zehnder shape, and thereafter, is heated at the required temperature for a required period of time to be thermally diffused. This optical waveguide 110 comprises: an incident waveguide 111; a branching section 112; branching waveguides 113a and 113b; a multiplexing section 114; and an emission waveguide 115, and a coplanar electrode 120 comprising a signal electrode 121 and an earth electrode 122 is disposed along the branching waveguides 113a and 113b. In the case where the substrate 101 of Z-cut is used, in order to utilize a change in refractive index due to an electric field in a Z-direction, the signal electrode 121 and the earth electrode 122 are arranged respectively just above the branching waveguide 113a. Further, in order to prevent lights being propagated through the branching waveguides 113a and 113b from being absorbed, the signal electrode 121 and the earth electrode 122 are formed on the substrate 101 via a buffer layer (not shown in the figure) consisting of oxide silicon (SiO2) or the like.
In the case where the conventional Mach-Zehnder optical modulator as described above is driven at a high speed, the signal electrode 121 is earthed at one end thereof via a resistor (not shown in the figure) to be made a traveling wave electrode, and a high frequency electric signal S, such as a microwave or the like, is applied from through the other end of the signal electrode 121. At this time, each refractive index of the branching waveguides 113a and 113b is changed due to an electric field generated between the signal electrode 121 and the earth electrode 122. Therefore, a phase difference between the respective lights being propagated through the branching waveguides 113a and 113b is changed, so that a signal light whose intensity is modulated is output from the emission waveguide 115.
For the conventional Mach-Zehnder optical modulator as described above, it has been known that an optical response characteristic of broadband can be obtained, by changing a cross section of the signal electrode 121 to control the effective refractive index of the electric signal S, and by matching a propagation speed of the light and a propagation speed of the electric signal S with each other. However, the electric signal S being propagated through the signal electrode 121 has a problem in that a propagation loss thereof becomes larger as a frequency thereof becomes higher, and therefore, the modulation band for the signal light is restricted resulting in the difficulty of high speed modulation.
As a conventional technology relating to the broadband of the Mach-Zehnder modulator, as shown in FIG. 8 for example, there has been proposed a configuration in which, in an interacting portion of the light and the electric signal S, to a polarization direction of a certain length part of the substrate 101 from an input side, a polarization direction of the remaining part (surrounded with a broken line in the figure) is inverted (refer to Japanese Unexamined Patent Publication No. 2005-284129). According to this configuration, if the modulation in a non-inversion region where the polarization direction is not inverted is performed in a forward direction, the modulation in a direction opposite to the forward direction is performed in the polarization inversion region. As described in the above, since the loss of the electric signal S becomes larger as the frequency thereof becomes higher, the intensity of inverse modulation in the polarization inversion region is high at the low frequency, while being low at the high frequency. As a result, the modulation at the low frequency can be suppressed in the entire optical modulator, and therefore, the modulation band becomes broader.
Further, for the conventional Mach-Zehnder optical modulator, there has been known a phenomenon in which a wavelength of the light is fluctuated at the modulation time (wavelength chirping), and this wavelength chirping problematically causes the distortion of the waveform of the signal light after transmitted through a fiber.
Briefly explaining the wavelength chirping, in the optical modulator shown in FIG. 7, one of the two branching waveguides 113a and 113b is arranged below the signal electrode 121 and the other is arranged below the earth electrode 122. Under the signal electrode 121 and the earth electrode 122, the orientations of electric field are opposite to each other, so that the refractive index change of the one branching waveguide is positive while that of the other is negative. As a result, the phases of the respective lights being propagated through the branching waveguides 113a and 113b are changed, and when the respective lights are multiplexed, the intensity of the multiplexed light is modulated. However, an absolute value of the refractive index change at this time under the signal electrode 121 becomes larger than that under the earth electrode 122, resulting in that the phase modulation remains in an output light. This is the cause of the wavelength chirping.
As a conventional technology for suppressing the wavelength chirping as described above, as shown in FIG. 9 for example, there has been proposed a configuration in which the center of the interacting portion of the light and the electric signal is made to be a polarization inversion region, and the length of the waveguide passing through the polarization inversion region is made equal to the length of the waveguide passing through a non-inversion region (refer to the pamphlet of International Publication No. 2004-053574). In this configuration, an integral value of each of the refractive index changes in the branching waveguides 113a and 113b realizes the zero-chirping, since the absolute values of the refractive index changes become equal to each other. Further, there has also been proposed a configuration in which the signal electrode is branched into plural numbers (refer to Japanese Patent No. 3695708).
Moreover, as a conventional technology for realizing both of the above broadband and the zero-chirping, as shown in FIG. 10 for example, there has also been proposed a configuration in which a polarization inversion region is disposed to perform the inverted modulation, to thereby achieve the broadband, and also, a signal electrode is branched into two to be symmetrically arranged, to thereby realize the zero-chirping (refer to Japanese Unexamined Patent Publication No. 2005-284129).
However, the conventional Mach-Zehnder optical modulator of the configuration as shown in FIG. 10 has a problem in that it is difficult to perform the impedance matching before and after the signal electrode is branched into two. Further, since two terminal circuits for the signal electrodes are necessary, the configuration of the Mach-Zehnder optical modulator becomes complicated, resulting in implementation drawback.